1. Field of the Invention:
The present invention generally relates to optical discs and an apparatus for reproducing signals recorded on optical discs. More particularly, the present invention is concerned with an optical disc in which signals for canceling a cross talk are recorded on a plurality of signal tracks, and an apparatus for reproducing signals recorded on such an optical disc in which a cross talk is canceled using the signals read from the plurality of signal tracks.
2. Description of the Related Art:
As one of various types of optical discs, there is an optical disc in which a signal pit string formed on its signal recording surface is spirally arranged from an inner side of the optical disc to an outer side thereof. A plurality of tracks are formed by a plurality of turns of the signal pit string. In order to enhance the recording density and thus obtain a large storage capacity, several approaches may be taken. For example, the recording density can be improved by arranging an increased number of pits in a direction of tracks. It is also possible to improve the recording density by narrowing the size of a laser beam used for reading information recorded on the optical disc. It is also possible to increase the recording density by decreasing the track pitch.
The track pitch has a limited value dependent on the size of a light spot converged on the optical disc. If the track pitch is decreased without a change in the spot size, signals recorded on adjacent tracks will be read together with a signal recorded on the target track. Hence, the cross talk increases and it becomes difficult to reproduce only the signal recorded on the target track.
In order to reduce the influence of the cross talk as described above, the following method has been proposed. That is, three light spots are concurrently projected onto three respective adjacent tracks, and signals are read from the tracks by using lights reflected thereby. A cross talk mixed with the read signal of the center track is tried to be cancelled by using the read signals of the other two tracks. For example, Japanese Patent Application Laid-Open Publication No. 3-40225 discloses a cross-talk cancelling method using a variable frequency filter. In this method, a frequency characteristic (filter coefficients) of the variable frequency filter is varied to find an optimum point to reduce the cross talk component in the output signal thereof. Another cross-talk cancelling method has been proposed in which cancelling coefficients (filter coefficients) are calculated so that the signals read from the adjacent tracks would have no correlation (see "NIKKEI ELECTRONICS", Nikkei BP company, No. 495, Mar. 19, 1991, pp. 98-99).
However, the above cross-talk canceling methods have a disadvantage in that it takes a long time to obtain optimal filter coefficients and thus it is difficult to rapidly respond to a change in the state of the optical disc. Further, the above-mentioned cross-talk canceling methods are not capable of eliminating the influence of an asymmetrical aberration of a lens system (coma aberration) and a warp of the optical disc. Hence, in these cases, it is difficult to effectively reduce cross talk.